pumice

A cellular or highly porous volcanic glass used in
"stone" form to smooth vellum and parchment for
writing or when preparing them for re-use, and
also to smooth the gold cushion. It is also used
in very finely powdered form in the manufacture of
vellum and parchment. It is composed largely of
aluminum silicate. See also: PALIMPSEST ; POUNCE (1) . (130 , 198 )